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Why California's Wildfires Are Most Destructive in Fall

Jonathan Erdman
Published: December 5, 2017

The recent firestorms that swept through Ventura County in December and Napa, Sonoma and Mendocino Counties in October, are only the latest examples of why fall is a peak time of concern for California wildfires.

Entire neighborhoods were leveled by the 1991 Oakland Hills firestorm, California's most destructive single wildfire on record.(Cal EMA)

Hurricane season may be over, and, for most in the U.S., it's still too early for major winter storms, but fall in California has a destructive reputation.

Including the October 2017 Tubbs fire in Sonoma County, California's most destructive on record, eight of the top 10 most destructive wildfires of record in California occurred in October, according to Cal Fire.

Seventeen of the top 20 most destructive fires on that list have occurred in the fall months of September through November.

The early December 2017 Ventura County wildfire - the Thomas fire - has many of the same factors in play as those destructive September-through-November events.

The Tail End of the Dry Season

Unlike most locations in the Lower 48 States, California has pronounced wet and dry seasons. It just so happens the last weeks of the dry season happen in fall.

"California’s Mediterranean climate makes it unique as our warmest months coincide with our driest," said Jan Null, certified consulting meteorologist and adjunct professor in the Department of Meteorology and Climate Science, San Jose State University.

Average rainfall in Downtown Los Angeles in the wet season (November - April) compared to the dry season (May - October). (Data: NWS)

Both Los Angeles and San Francisco typically pick up only 9 percent of their average annual precipitation in the six-month "dry season" from May through October.

This amounts to a six-month average rain total of only 1.3 inches in L.A. and 2.25 inches in San Francisco.

"By the end of the summer and into early fall the state’s vegetation has dried out," Null said.

In 2017, the dry weather lingered into the typical wet season in Southern California.

Only 0.01 inch of rain was measured in downtown Los Angeles in November, their driest November since 2000 and over an inch less than their average monthly rainfall of 1.04 inch.

This left vegetation "parched and primed to burn with even the slightest ignition source," according to a National Weather Service-Oxnard forecast discussion from Dec. 4, 2017.

"Recall we had nearly five years of extreme drought followed by record rains last winter that produced a bumper crop of grasses and fine fuels on top of drought and disease-stressed heavier fuels," wrote the Monterey, California, National Weather Service office in an Oct. 9, 2017 forecast discussion following the Napa/Sonoma firestorm.

"Fuel analysis just ahead of the winds showed they were at all time record dry levels preceding the front."

Which leads us to the main driver of California's fall firestorms.

High Winds Return to Fan the Flames

Summer's wildfires in California tend to burn more slowly, starting in more remote areas, often due to lightning.

In the fall, Calfornia's infamous offshore winds, known locally as Santa Ana (southern California) or Diablo (northern California) winds, typically kick into gear, according to a 2017 climatology study.

A typical Santa Ana (or Diablo) wind set up in California.

"In September and October, we begin seeing high pressure developing over the Great Basin and this creates dry warm offshore winds," said Null.

"Since the Great basin is nominally 4000 feet in elevation, the air is compressed as it descends to sea level, warming and drying it. When this flow is forced over mountains and through canyons it accelerates."

If the jet stream is also located just to the east of the state over the Great Basin, a downward push of strong winds can occur, intensifying the offshore wind event.

These intense offshore winds can occur, at times, from fall through spring.

What makes them particularly dangerous in the fall is that they occur when, as mentioned earlier, soil moisture is at its driest after the dry season.

These Santa Ana or Diablo winds can quickly whip either an existing wildfire or just-developed small brush fire into an inferno, blowing embers downstream, starting many more spot fires, often in more heavily-populated areas.

They can also persist for several days if the jet-stream pattern, featuring blocking high pressure over western North America, persists, as has occurred in early December 2017.

Jonathan Erdman is a senior meteorologist at weather.com and has been an incurable weather geek since a tornado narrowly missed his childhood home in Wisconsin at age 7. Follow him on Facebook and Twitter.

The Weather Company’s primary journalistic mission is to report on breaking weather news, the environment and the importance of science to our lives. This story does not necessarily represent the position of our parent company, IBM.

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